1. Field of the Invention
The present invention relates to a wireless communication system and communication equipment, and more particularly to the frequency hopping wireless communication system which is arranged so that a base station can manage communication control timings and transmission rights for plural communication equipments and the wireless communication equipment which applies to the system.
2. Description of the Related Art
There has been conventionally proposed a wireless communication system which is arranged to communicate data between wireless terminals through the application of "a frequency hopping spread spectrum" for changing a carrier frequency for each cell formed by a base station, based on a predetermined same hopping sequence and hopping period. As such a wireless system, a system for hopping a carrier frequency at each wireless communication frame (called as a prior art 1) has been known in IEEE 802.11 Working Document, IEEE P802.11/92-39, "Medium Access Control Protocol for Wireless LANS" (Publication 1), for example.
The prior art discussed in the publication 1 describes that a wireless communication frame (simply called as a communication frame) is composed of a first field used by the base station for broadcasting data for all the communication equipments (mobile wireless terminals) included in the cell, a second field used by a wireless terminal inside of the cell for sending data for the base station though the data may conflict with another piece of data sent by another wireless terminal inside of the cell, and a third field used by a wireless terminal inside of the cell for sending data according to an indication given by the base station. The length of each field is adjustable to each kind of system. The information indicating the length of the field is placed in the first field of each frame.
The base station transmits control information indicating a carrier frequency of the next wireless communication frame by the first field of the current communication frame, so that each wireless terminal inside of the cell hops the carrier frequency at each frame. The publication 1 suggests that a carrier frequency possibly conflicts with another carrier in an overlapped portion between the adjacent cells if any.
On the other hand, as one system for accessing a wireless channel, there has been conventionally known "split-channel reservation multiple access method". In this method, a wireless communication frame is divided into a control information transfer field and a data transfer field. The control information transfer field is divided into a transmission request field composed of plural slots and a reply field for each transmission request field. Each wireless terminal requests a data transmission right of the base station through the transmission request slot and the base station gives back the reply (transmission right) to the request through the reply field.
The concrete arrangement of the foregoing split-channel reservation multiple access method is discussed in "Access Control Method for Wireless LAN" in IEICE (Institute of Electronics, Information and Communication Engineers) Technical Report, RCS 92-37 (Publication 2) or the foregoing publication 1.
As indicated in the publications 1 and 2, a wireless terminal trying to transmit data inside of the cell to which the terminal itself belongs or another wireless terminal trying to register for the cell to which the terminal does not belong uses any slot defined in the control information field of each communication frame for issuing a transmission request or a registration request to the base station through the effect of the Slotted ALOHA method (called as the conventional system 2).
Further, encipherment of data to be transmitted on a wireless channel and obtention of security of data by compressing the transmission data (called as the conventional system 3) has been known in Nikkei Communications No.148, 1993, 30 to 31 (Publication 3).
In the frequency hopping spread spectrum system described in the publication 1, if the adjacent cells partially overlap with, the received data may become erroneous, because a probability exists that the carrier frequencies may conflict with each other in the overlapped portion. The error rate of the received data is presumed to be about 0.5.
In this case, the use of an error correcting mechanism disallows correct data communication. Hence, the communication efficiency is made lower by a conflict time against the overall communication time.
The probability that the foregoing conflict of the carrier frequencies takes place in each cell is, in general, made higher in proportion to the number of adjacent cells and in inverse proportion to the number of hops of a carrier frequency in a hopping period. For example, the use of a Read-Solomon sequence as the hopping sequence, in any two sequence, allows the carrier frequencies to conflict with each other once at one period at maximum. Hence, if interference takes place among n cells, each cell of which uses n frequency hops, the carrier frequencies may theoretically conflict with each other in all the time zones.
Further, if as described in the prior art 1 the system for reporting a next carrier frequency to be hopped in each wireless communication frame brings about a transmission error, thereby disabling to receive the carrier frequency information, disadvantageously, each wireless terminal disables to correctly receive the subsequent communication frames unless the communication frame is re-synchronized.
On the other hand, in the prior art 2 arranged to use the Slotted ALOHA method for the control information field of each wireless communication frame, each wireless terminal enables to transmit data having any content to any slot. Hence, no restriction is placed on the number of the wireless terminals to be located in each cell. It means that only that slot is used for the obtention of the transmission right and the request of registration for the cell. The use of the Slotted ALOHA method, however, involves the following inevitable disadvantage: That is, the increase of the wireless terminals for transmitting data to a slot (enhancement of a utilization rate of the control information field) leads to the increase of the re-transmissions resulting from contention of transmissions from plural wireless terminals on the same slot, thereby lowering a throughput.
Further, compression of the transmission data is realized by converting the transmission data train into another data train having a small amount of information according to the predetermined conversion rules. As the conversion rules for data compression, the public rules are applied. Hence, only the application of the prior art 3 disallows data transfer efficiency to be improved and positive security of transmission data to be obtained.